The problem is it seems far-fetched even at that. The ionization chamber where the Americium is stored should be shrouded by metal and that would completely stop alpha emissions. I'm not saying that couldn't be leaking, just that the way smoke detectors are designed you shouldn't be exposed to any radiation (and even if you are, you pretty much need to swallow the Americium to be at any risk). Another issue is Geiger counters usually don't detect alpha emissions. Given that this was a storage site filled with what probably was mainly alpha and beta emitters (radioactive decay likely removed most of the gamma emitters), it is possible they had a Geiger counter for detecting alpha-beta-gamma emissions. The vast majority of Geiger counters only detect gamma emissions, though.