Like in the below, if you simply want to say that you've forgotten(left behind by mistake) something, which article would be correct to use?
I left my purse on the/a bus.
i left my bag in the/a cab.
In these cases, both 'a' and 'the' may mean 'on one bus' and it does not matter which. 'the' may mean 'the bus', the referent of whose is important and is known by the speaker and the hearer. 'a' could mean I traveled on several buses, and my wallet is now in one of these buses, but I do not know which, or I do not want to specify for the hearer which.