"from the other side of a glass door"?
I think TB means in airports in places such as the States, when inbound and outbound crews can't physically meet. If the incoming flight is late and the outbound crew get to the gate on schedule (because the company "forgot" to delay their pick up from the hotel
), you'll perhaps see your colleagues on the "otherside of the glass door" which separates arriving from departing pax and crew at the gate.
"So, I guess the only colleagues you would bump into at the hotel are those who are not flying the aircraft back that you just landed".
The aircraft are not always turned round in minimum time ( e.g. departure might have to fit in with home base ATC slots timings), the inbound may arrive early, the drive from the airport might be short.... there are all sorts of variables mean that sometimes crews "cross" in the hotel lobby, but it's not the norm.