In this case, I think you - as a developer - need to decide on “how long is too long”. Let’s say there is a threshold time t=24 hours after which it is safe to assume that slot needs to be cleared. This is how I’d work with this:
As soon as the Agent receives a new message, check the timestamp of the previous message. (This timestamp can be obtained from the Tracker of your bot.) If the previous message is older than t (which is 24 hours in our example) you can simply reset the tracker. Resetting the tracker will clear its entire history.