I came upon this gem today and was utterly amazed! I can imagine the conversation from the tech and his helper after they finished changing the compressor. "Hey Billy, we are missing a cap, lemme run down to the truck and grab a new one". "Listen here youngbuck, we don't have time for that. Lemme show you a trick that works wonders for leaky schraeders!"
Yes, it is what it looks like. A rubber plug from the new compressor was slathered in leaklock and firmly placed over said leaky schraeder. I was amazed. This whole time I have been throwing them away thinking that they were useless.