“They asked, ‘Did you hear people speaking Russian in the Trump office.’ “

“Who?”

[ … ]

“Did you hear about Trump Tower-Moscow?”

OK. This is significant. Prosecutors ask questions based upon things that other witnesses have told them - or that they have garnered from wiretaps.

This tells us that
1. Mueller has evidence that there were Russians present in the Trump campaign offices, and is looking for confirmation.

2. Mueller is looking into the promised trade-offs between Putin for a sweetheart deal on a Trump hotel in Moscow.

The money. The money is going to be what brings this fucker down.