Submarines cannot 'see' aft so if one was lurking in the Baltic and was emerging it might not have seen the Estonia coming towards it from behind. Maybe it was a Russian sub after all and Bildt wanted to avoid an incendiary situation in Estonia, with Russia feeling it needed to bring in 'peace-keeping troops' and never leave.
Fact is, the Russian Navy put out two official disclaimers saying none of their fleet was in operation as of the time of the accident, yet, the Finnish coastguards had to intercept a Russian vessel the Leonid Bykof. heading towards the rocks within one hour of the accident, so can't believe every statement issued, which appear to be more to do with PR and reputation management than factual truth.