D&D 5E - Detect Thoughts spell doubt: can I concentrate on the target without going deeper into its

"As an action, you CAN either shift your attention to another creature's thoughts or attempt to probe further."

Nothing in the text says you "MUST USE YOUR ACTION" to shift your attention...

So, I interpret it this way: You can read surface thoughts...and choose not to use any action at all and just stay on one person...

If you so choose, you may use an action to do something different. The spell stipulates what That action can do: change target or probe deeper.

I'm not saying DEFCON is wrong. I just read it differently. Your table has to decide how to play it.

Edit:
And wording aside, my reasoning for not forcing someone to have to move it is this: If you are forced to move it, a player is just going to keep moving it to and fro between the target subject and another random person, perhaps even an ally. So, for an interrogation which is measured in minutes (or hours)and not 'rounds', it's kind of a moot point whether you spend a 6 second action to move it one place and back again.

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