How does UKVI know your travel outside UK dates?

This is more a curiosity issue as I am filling the SET form but UK only registers entrance to the UK not leaving.

airlines share your data with them.. they know exactly when u enter/leave.

Do you know this as a fact or is this an assumption (nothing wrong with that this board is full of ssumptions)

I highly suspect that airline sharing leads to a comprehensive exit data. For example multiple passport flyers, correlating to such data (airlines do not check enterance visa only exit visa to destination)

Eh? UKVI agents check at entry and airlines check at exit. Add 2+2 together.

seasky wrote:Do you know this as a fact or is this an assumption (nothing wrong with that this board is full of ssumptions)

The truth is out there (well some of it at least).
The only assumption is you can google for it.

There is an exit policy.