Is this sentence correct?

Anonymous Look around before you leave the bus cause you may have left something.

because

Can I also use might have and could have instead?

Yes.

You should put an apostrophe before cause ---> 'cause.

moginnYou should put an apostrophe before cause ---> 'cause.

Except in the most informal writing, the word should be written in full. In very informal writing, it's more likely to be written as coz than 'cause.

Can't since or as be used as substitutes?

AnonymousCan't since or as be used as substitutes?

http://dictionary.cambridge.org/grammar/british-grammar/as-because-or-since