Time outside of the UK - PR

[LilyLalilu]

I have a period of 3 months during my 5 years of treaty rights where I was outside of the UK travelling.
Afterwards I returned to the UK and was self-sufficient for a while before starting a new job.
I got CSI from the date that I returned to the UK but did not have it while I was travelling as I did not need to exercise treaty rights because I was not in the UK.

My question: Will I need to prove that I was actually outside of the UK for these 3 months or will they just take my word for it?
Just asking as they could obviously think that I am just claiming to have been outside of the UK as I did not have CSI during that time.

I lost my passport from that time but my bank statements show some cash withdrawals from overseas cash machines and I also have my plane tickets - would this be sufficient?