Yes ! We found it was some flourescent lights and not the electronic (high frequency) ones - but not all of the standard choke / starter ones either! Find a position for the probe where it produces a reading and then just cover it up so it's in the dark and I bet the signal stops.
We think its a beat frequency between the flourescent lamp and the sampling rate of the pulse oximeter. It probably only happens with some tubes because they have a particularly long or pronounced "off" period.

Today is the day you worried about yesterday - and all is well !