Anything to be worried about?

[Delphinus]

I'm assuming this is a new tank build by Concept hence it's being posted in this particular forum.

To me though it looks like condensation between the glass (on the outside) and the trim. If you had it filled with FW, odds are the water was cold and condensation formed on the glass. This trickled down into under the trim area on the bottom. Then the tank was drained, the glass got back to room temperature, the condensation evaporated off the glass except for where air circulation is poor (ie under the trim) and thus it's lingering there.

Water trapped in the trim area can take a long time to dry out.

Just a theory though.

The true test of a leak is how wet things get under a tank. If there isn't a puddle or trickling down the stand, then I'm thinking it's just condensation.

You could always fill it back up (at least partially) with water and see if any wetness forms under the tank..