If the POV character can hear them, it's fine, yes. I would still hope that the POV character in some way cares about them or reacts to them--simply using the POV character as a microphone seems to be slightly misusing the concept.
Thanks @ChickenFreak, I think I agree, hence the reason I brought it up in the frst place; it does feel slightly awkward. I may try and include the POV character in the conversation. It currently happens within earshot, but it has no relevance to them. If I include them in the conversation, at least they are invested. Their opinion on the subject may also give me opportunity to develop them a little more too. Thanks.
1. If the POV is hearing them, and you intend for him to hear them so HE learns something he didn't know, use dialogue... he is eavesdropping, maybe, or a name or a phrase catches his attention to make him listen to something he would otherwise ignore. 2. If the POV can't hear them, but the reader needs to know what was said, then either a: synopsize them in narrative or b. do a POV shift to either one of the talking characters (if it is important and long enough to warrant) or to third omni. I recommend the latter, the reader is seeing/hearing them talk, but is in no one's head unless there is something unheard inside the speaker's head you need to convey. 3. If neither the reader nor the POV need to know what is said, dump it... it is filler.
If you're using a close 3rd POV, then the caveat is that you'd have to write the conversation from the POV of the POV character. Simple enough to do, but it's also easy to slip up and occasionally jump into the head of one of the people actually involved in the conversation.
Depending on how many of these overheard conversations you have, you could also look at the question of whether this conversations have a common thread and use a character in that thread as a minor POV. I have one POV whose primary purpose is to give us a window on what the dark side of my story is up to when the good guys arent around. If you have someone like that, that make fun POVS to write. Mine is a reporter at my fictional TV network who has curried favor with the evil editor and is roommates with villain's minion secretary - so she's not important to the villain's plans {yet} and is at best a secondary minion, but she's compelling in her own right, likeable, and fun to watch while still being on the wrong side.