I think it's very very rare nowadays to hear 2 pilots chatting on 121.5
Most unauthorised transmissions fall into 2 categories:
1) The poor chap who has just made a mistake; eg trying to call his company on 121.5 by mistake. When he gets no reply he will check the freq, be a bit embarrassed, and select the correct freq. We've all done it at some time or other and this Chap doesn't need a "your on guard" admonishment.
2) The moron, probably and hopefully not a pilot, as previously described who gets a kick out of winding everyone up by transmitting stupid noises. Any response to this idiot is also inappropriate because he's a wind up merchant and it will just encourage him to keep doing it.
So, please, NO unauthorised 121.5 transmissions - the "you're on guard" call is in fact worse than the original transmission because you know you're doing it.
Just keep quiet - that is staying professional!