It's most likely not missed RTS. Make sure your team is putting separate labels on everything and not hitting reprint. (Seriously, check random hold spaces once in awhile. You shouldn't be surprised what fulfillment TMs will do to cut corners when no one checks on things preemptively.) Also, make sure that the drive up team is grabbing everything and not just robo scanning and forgetting to grab some bags. That'll cut down on random things left in your hold locations. Missed RTS is rare, if it even happens at all anymore.
Assign a TM or yourself to check every bag in the hold area once a week. Do it at a slow time of the day for orders and when the holds aren't overflowing. At my store Sunday or Monday early morning works well. I just go through and scan by date. If it's more than 3 days old in the coolers or freezer, out it goes. Since we put all groceries in the dry space, even those that come in regular batches, I just pull everything over a week old. Anything without a pick label gets a sticky note to remove it the next week. I'm just a TM, so I really don't care why old things are still there. I just want them gone, so going by date works for me as my TL couldn't care less.
That works well as long as I have a chance to do it every week. If I don't get to it for awhile because I have other shit to do, then well that's when I find really nasty things in the coolers.