No. It could either be "if they were seen" ( in the present) or "if they had been seen" in the past.

Thanks Kenny!

The police would follow someone if he was seen a robbing a bank.( unlikely condition > as a warning) Someone is singular , and "he" is the pronoun if the singular indefinite pronoun(someone) gender is not known. The police would have followed someone if he had been seen robbing a bank > an imagined situation in the past .