This sounds like a low battery alarm (not sure how wedging the keys might have silenced it though) - the 12V sealed lead-acid battery is just about old enough now to start failing. When the backup power drops below 11V, the chirp will start, indicating the need to replace the battery. Find the battery and disconnect it completely, it should stop. Basically, all these years the battery recharging circuit was live, keeping the battery charged - pretty common with abandoned alarms.
Additionally, if the chirps are not from the local sounder, smoke alarms hooked up to the system may also start chirping on their own if their local 9V batteries are dead and the system no longer supplies enough voltage centrally. In other words, be prepared to replace many of the smoke alarms batteries at once when you disable the larger 12V battery. The smoke alarms will have to start relying on their own 9V batteries and they are most likely dead by now.
- posted 7 years ago