Ok, I got my mirror back and here is how it worked: I sat in the car just about to drive away, when one of the hotel guys knocked on my window. His colleague would know where to get another mirror, if I were interested? Of course! What followed was a drive across town thru lively quarters. When we passed the market, he told me to slow down. Then the guy with the light green t-shirt in front of the gate approached us waving: he would have a left rearview mirror for a Nissan Tiida; they were hard to get; - 120 soles? (~50 bucks). In the meantime two other guys appeared, immediately checking the retainer for fitting and showing not one, but two mirrors.
I offered 100, we met at 110: 100 plus 10 for the installation. - My take of what happened: when I arrived the night before at the hotel, I was negotiating a better price. (125 instead of 195 soles). I then brought the car to a cochera from where I got it in the morning and parked it in front of the hotel, which was literally 5 feet away from the entrance. I then had breakfast and returned to my room to get my stuff and to take a leak, enough time for the thieves to make it happen. The question is whether they have been informed by the hotel staff? I indeed think so; why would the hotel guy have otherwise be able to point me to the presumably only place in Trujillo where to get a mirror? And where we were awaited already? Conjectures, but this game was rigged.
Also I swallowed the pill, first and foremost since I need the mirror every other second while driving in city traffic and secondly to buy one would have been quite a hassle.