There won't be any water in the tray since the shower valve is OFF.
I think bhg is right about the flow switches. Might just need adjusting, or possibly replacing.
There's a remote possibility that there is air in the system, especially as it has recently been disturbed. With a bubble of air in there, the switch might sense the lack of water and "think" that it needs to come on.
Using the shower a few times should clear it.
Otherwise...
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