At the time of observation in 2020, these flies were rapidly increasing in numbers around the bee hotels. It is a fruitfly species, Cacoxenus indagator.
The flies are cleptoparasites on certain bee species that nest in the bee hotels like the Red mason bee (Osmia bicornis) and the European orchard bee (Osmia cornuta). The fly larvae eat the food for the bee larva, but if enough food is left the bee will still develop although the emerging adult will be smaller in size.
The flies spend a lot of time underneath the bee hotel blocks where they fly in a bouncing manner, to and from the surface but without touching it. Then they come to the front of the block where the nest entrances are and start to observe the bees or inspect a nest entrance by entering it.
References1 Nederlands Soortenregister
2 KRUNIC, M & Stanisavljević, Ljubiša & PINZAUTI, M. (2005). The accompanying fauna of Osmia cornuta and Osmia rufa and effective measures of protection. Bull. Insectol. 58. 141-152.
3 MOENEN, Rosita. De broedparasiet Cacoxenus indagator (Drosophilidae) en de parasitoïden Melittobia acasta (Eulophidae) en Coelopencyrtus sp.(Encyrtidae) bij solitaire bijen in kunstmatige nestgelegenheid. entomologische berichten, 2012, 1-2.