Tyre valves were a bit loose

[M5Rick]

Check that tyre valves are snug in with a tyre valve tool, all mine needed a final eighth or so turn when pressures in all four tyres lost about 6 psi each.
I use M metal valve caps and tighten further a little with mini 9mm spanner so they can't be undone by hand.

[M5_Raz]

I’ve lost some metal caps over the years, always thought people were stealing them.

Seems like the vibrations of day to day use is causing them to come undone and come off.

[M5Rick]

When the M on the valve caps gets cloudy I have a fresh set to replace.