I'd guess that about 95% of the time a pressure loss is caused by items attached to the air compressor rather than the compressor itself.
I own four compressors and all of them can be unplugged and they will continue to hold pressure for months when not in use. But, they all tend to lose tank pressure in a day or two when I leave all of my attachments hooked up to a compressor (dial regulator, air drier, hose reel, hoses, quick connects, tools, etc.)
I partially solved that problem by attaching a "full port ball valve" as the very first attachment to each air compressor's output. That is followed by a quick connect fitting. This gives me the capability of verifying that a compressor tank is holding pressure. It also allows me to shut off the compressor outputs when not in use - thereby allowing me to hold tank pressure since I tend to procrastinate tracking down slow leaks in attachments.
Obviously, components should be reasonably good quality, should be attached with Teflon-style tape, and should be leak-tested.
FYI: Hose reels, especially inexpensive bargains versions, are often the source of pressure losses. O-rings should be lubed and/or replaced on a periodic basis.
Good luck, Gideon