On a Type O non immigrant visa, I've just done my 90 days reporting, if I needed to leave and come back; how do I go about informing the authorities please?
TLDR : Answer Summary
If you hold a Type O non-immigrant visa and have just completed your 90-day reporting, ensure you have a multi-entry visa or obtain a re-entry permit from an immigration office or international airport before leaving Thailand. When you exit, the immigration officer will take your departure card, indicating you're no longer in the country. Upon return, you will need to file a new TM30 form, and your 90-day reporting countdown will reset.