Regarding "my friend left the envelope on a table in his house" not producing the "AirTag found moving with you" alert, this is obviously by design. You get the alert if the tag is planted on your person/vehicle and following you around, not if it happens to be sitting still in a location that you frequent.
The alternative is being constantly bombarded with alerts because other residents of an apartment building also own AirTags, which is not going to be useful to anyone.
The alternative is being constantly bombarded with alerts because other residents of an apartment building also own AirTags, which is not going to be useful to anyone.