'They' arrived just before the Falcon and the 'Lando system' isn't that far.

So it would make sense he would jump to hyperspace to get there before Han.

If I were filming it from scratch I would have an indicator on the Falcon go off when Fett emerged from his Space Junk cover but it would be near the deactivated Threepio.

The irony being Han and Leia have just switched off the only 'person' who might have noticed Fett (someone they have already ignored as he tried to warn them about the hyperdrive and who will get blasted trying to warn them about the troopers).