When Sam returns to the hotel room and sees that Dean isn't there, he takes out his cell to call him. Sam then proceeds to scroll down the recent calls list to Dean number, you can see Dean was at the top of the list. Sam wouldn't have had to scroll down to the fourth call that also says Dean.
After Sam sleeps with Cara, he calls Dean, who now believes Sam to be under the Sirens spell. He tells Sam to stay put as he must now finish the job alone. However, the siren can only be killed by a blade dipped in the blood of someone under its spell. That would mean Deans best chance of finishing the job is getting to Sam and getting some of his blood, not staying away from him.
Since "strip clubs" have many, many mirrors it is inconceivable that the siren wouldn't be spotted while inside.