One step thought detection vs Two-step
The difference is that in order to detect the thoughts, you must first focus on a single creature. If you can see them, you can immediately identify and Detect Thoughts from the Cast a Spell Action of the initial casting(emphasis mine):
When you cast the spell and as your action on each turn until the spell ends, you can focus your mind on any one creature that you can see within 30 feet of you.
The spell also gives you an opportunity to detect thoughts of an unseen creature. You can either do so with the initial Cast a Spell Action (doing so replaces your initial chance to Detect Thoughts of a seen creature), or do it with your next action.
You can also use this spell to detect the presence of thinking creatures you can't see. When you cast the spell or as your action during the duration, you can search for thoughts within 30 feet of you.
This enables you to "find" the unseen creature, at which point you can then Detect Thoughts with the action outlined above.
- Initial Casting of Detect Thoughts - Choose between picking a seen creature and Detecting it's Thoughts OR Detect Thoughts of Unseen Creatures.
- If you Detected Thoughts of a visible creature, then that is your focus for the duration of your spell unless you use an Action to change your creature focus.
- If you chose to Detect Thoughts of Unseen, you can now identify either a seen creature or an unseen to Detect Thoughts. Your next Action can be to Detect Thoughts of a Seen or Unseen creature.
- You can choose to use a later Action to shift your Detection to another creature or probe deeper into their thoughts:
As an action, you can either shift your attention to another creature's thoughts or attempt to probe deeper into the same creature's mind [see spell description for mechanics of probing deeper].
This is all supported by Jeremy Crawford via Twitter:
In detect thoughts, it's 1 action to read thoughts if you can see your target, at least 2 actions if you can't.