Overseas assets, income or gains letter

Hi there, I am a UK citizen living abroad. I received a letter saying that HMRC has been informed through UK's tax information exchange agreements with other countries that I mat not have declared everything I need to in the UK. I am very confused by this as I can't see what this could be referring to. Is there anywhere I can get specifics or someone I can call for clarifications? Thank you for your help

Hi katquestion,
Please see guidance here:
Tax on your UK income if you live abroad
Non-UK residents: Income Tax and Capital Gains
Thank  you.