Could my dog have climbed into a plumber's van and left with him?

Call them today, don't wait. It's not silly at all, it's a completely normal question and dispatch gets stuff like this more than you'd think. When I ran service calls we had a lady call once convinced her cat ended up in our truck, and honestly the crew chief just checked the van and called her back in twenty minutes. Zero awkwardness. If Biscuit is in that van, the sooner you call the better, because vans get cleaned out, restocked, and reassigned constantly, and the tech might not even look in the back until end of week.

And yes, a 12 pound dog can absolutely disappear in a work van. Ours had floor mats, a stack of drop cloths, a couple of folded blankets under the equipment rack, and a cabinet bay that stayed open half the day. A friendly little terrier that's comfortable with strangers would curl up behind the water heater boxes and you'd never know she was there unless you did a full walkthrough. Doors open for five minutes at a curb while the guy loads up is plenty of time.

When you call, ask for the dispatcher rather than just whoever answers. Give them the invoice number, the date, and the tech's name if it's on there. Ask three things: whether the tech logged any other stops in your neighborhood that afternoon, whether the van has an interior camera (a lot of company vans do now), and whether you can leave your number in case anything turns up during their next cleanout. Also ask if they can just have the tech check the van today, not at the end of his route. Two more things that actually helped a friend of mine in a similar spot. First, get your neighbor's best guess on the van and see if anyone on the block caught a partial plate on a doorbell camera, since you'll want to confirm it matches your invoice company anyway. Second, call the shelters one town over in the direction the tech would have driven home, not just your local ones. If she did ride along and got out somewhere unfamiliar, she's probably close to wherever he parked that night. You're not being paranoid. Dogs ending up in trade vans happens more often than people realize, and the companies generally want to help.

Adding a small data point here since my sister's beagle pulled almost the exact same stunt with an HVAC van two summers ago. The tech never noticed her, and she only turned up because she started barking when he parked at his third stop of the day. So the dispatcher call matters, but also ask whether the tech can remember any stops after yours where he heard noise or saw the dog get out, because that is how these usually resolve. Also, terriers are burrowers by instinct, which is exactly why a soft pile of drop cloths would be irresistible to her, so I would not rule the van theory out at all.

We had something similar with our cat and an electrician's truck years back. Turns out he'd parked two streets over for his next job and she got out there. Took us three days to figure it out because we only searched our own block. So yeah, if she did ride along, don't just call the office, ask them where the tech's next few stops were that day and go walk those streets. Also toss an unwashed shirt of yours near where the van was parked. Familiar smell can pull a scared dog back to one spot even days later. And keep checking the shelter, she coud get picked up by someone totally unrelated to all this.

Before you go all-in on the van theory, walk your block and check open garages and sheds. My mom's little mutt pulled a disappearing act for two days and it turned out a neighbor had shut her in his garage without noticing and left for the weekend. Friendly small dogs follow people right into open doors and nobody clocks it. Also worth checking wheather anyone on your street had work done that afternoon too, since your neighbor's mystery van might not even be the plumber's.

First post here so sorry if this is random, but your story hit close to home. My beagle jumped into a landscaper's trailer once and we only found him because he howled at a stop sign three streets away. One thing that helped us was calling the office with a photo attached to an email, not just describing her over the phone. The receptionist forwarded it to the techs and one of them recognized her immediately. Also maybe worth asking if they have a shop or yard where the van parks overnight, since a dog could wander off from there too. Fingers crossed she just wandered into someone's garage like the other poster said.

Update: Called the office Friday and the dispatcher was surprisingly nice about it, no laughing at all. Driver checked his van and the depot, no dog. So at least that door is closed. The tip about widening the search past our own street got me thinking though. If she bailed out somewhere, how far does a small dog usually wander before someone spots her?