I mean the temperature/humdity sensors whose official name is “wireless tags” (wirelesstag.net). I have them working in HA, but we had a guy come out today to check on the dehumidifier in our basement, and the wireless tag down there was showing 50.2% humidity but his “professional” sensor, as well as the one on the dehumidifier itself, was at 38%. I certainly don’t expect perfect accuracy, but this is a very wide discrepancy.
I haven’t tested mine against a professional device, but I find they are pretty consistent when compared with one another. Positioning is important, I find - mine are mostly on doors and windows to detect movement so they give a higher humidity reading than one in the middle of the room. You might try looking at the stats as well as the current reading in case there was a blip caused by air currents etc.
I seem to remember something about humidity calibration in the wireless tag app, but I’ve hever used it, and the documentation is a bit opaque.