Flies do not interest me in particular, but I noticed this one because she (?) shadowed bees flying around the bee hotel near the ground at close distance. When the pursuit was broken off she moved back to an lookout position and waited for the next bee to pass by.
It turns out to be a Anthomyiid fly from the genus LEUCOPHORA, which is a kleptoparasitic group of flies on certain bee genera, o.a. ANDRENA, COLLETES  and HALICTIDAE . The referenced species in literature are all ground nesting bees.
The females follow the bee towards the nest and wait for the host to leave the nest for foraging pollen, then enter it and lay one or more eggs near the food collected by the bee.
This fly followed male bees of the species Red mason bee (Osmia bicornis). Probably because they are abundant and very busy all around the bee hotels, so this could be a situation of mistaken identity.
She follows the bees in a space of up to about 1 meter in height.
I’m going back to observing the fly to see whether she will follow a female bee to the bee hotel nest.
Another specimen had positioned herself in a lookout position on a garden tile from where she observed low overflying bees and every now and then followed on for a short while.
References1 LOONSTRA, Anne Jan. Waarnemingen aan Leucophora (Diptera: Anthomyiidae) bij bijennesten. Bzzz/HymenoVaria, 2010, 31.1: 22-23.
2 Polidori, Carlo & V, Michelsen & Nieves-Aldrey, Jose-Luis. (2015). Leucophora Satellite Flies (Diptera: Anthomyiidae) as Nest Parasites of Sweat Bees (Hymenoptera: Halictidae) in the Neotropics. Neotropical Entomology. in press. 10.1007/s13744-015-0301-x.