What Brompton do you have? Mine sometimes squeaks from the cogs/chain tensioner after lubing the chain. Maybe try spinning the cranks without the chain attached to the chainring and see if it does it then?
S6L. Defo not them. Chain gets cleaned and relubed, also relubed my chain tensioner about a year ago, and replaced it more recently. Cranks/chainings could be contributing to the problem too though.
What Brompton do you have? Mine sometimes squeaks from the cogs/chain tensioner after lubing the chain. Maybe try spinning the cranks without the chain attached to the chainring and see if it does it then?