So: Since I do ever since have a strikingly similar issue with sallies (larger plants, cuttings are clean probably because of proper sanitisation) I've made use of a handy situation I currently have: For winter I've out one sally in a room where there is no other plant.
So yesterday night I went ahead and took out the old yellow and blue glue cards and set one of each up for it. I was a bit worried the glue turned bad after maybe 10 years or so. Nope, guess what: First catch this morning on the blue card. A tiny fly it seems... well it has wings you know. I'll try to get means of magnification and out a picture up later.