Confused, need outside advice!

My partner of almost 12 months, is separated (married but not living together), and has been for almost 2 years.

My partner has informed me that they slept together last night when having visitation with their child.

Apparently it was because the ex hadn’t slept with anyone since their separation and they both didn’t say no to that urge. And my partner reassures me it isn’t meaning they’re getting back together.

Do I just walk away now and let them work this out? Surely if my partner was over the ex, this wouldn’t have happened at all.?