Tyre pressure monitor

Was out on a jog down the M6 yesterday when our pleasant cruise was interrupted by the beep/bong of the warning system and a warning that it had detected. drop in tyre pressure. Flipped on the vehicle status monitor and the tyres were flagged up in orange. Oh dear...

While my other half was having palpitations (she's experienced a proper 70mph blowout before), I was scratching my head wondering what the problem was when there was no change in steering or ride. Still, we were close to a junction so off I went to check it out.

No visible problems on any tyre, hooked up the compressor & all four tyres were down to roughly 28psi (just below 2 bar). Clearly below normal, but there was no sogn of a sudden drop. Inflated them back up to recommended and a day later they've kept pressure.

So a bit puzzled as to what happened, though I don't know how these monitors work.