When Glenn tries calling Garrett from inside the "haunted house" to let him out, Garrett declines the call and Glenn is immediately heard leaving a voice mail. There was not enough time for him to be sent to voicemail, hear the brief message from Garrett and/or his service provider, wait for the beep, and then start talking.
After Dina discovers that Maya took the goody bag containing the union cards from the Fenton store, Amy says that Maya left about 20 minutes earlier and would be long gone. Dina explains that the security footage shows that Maya stopped and spoke to Brett, then did some grocery shopping, and thus had only just left the store. Amy and several other employees were stationed by the door to check all of the outgoing goody bags for the cards. There's no way none of them would have noticed the new district manager leaving.