She is a thief who is hoping to get away with it
This was left as cash with no name or instructions. She took her wages. £50 is hardly difficult to count. Then she took £50 money left laying around in the half table. She knew exactly what she was doing hoping you wouldn't notice. It's dishonest, what else has she "lifted" from your house.
Contact the agency and ask them to contact her to call you
Then text her to say, you took your wages why did you take £50 I put in table as well? I didn't know my DH had left wages out. You need to return that.
I'd expect her to bring it back or to say that's next weeks wages so I don't expect to be paid for then
If she's self employed working for you (effectively the cleaning company are introductory agents) then you should have her details. That she's being paid in cash indicates possibility she's not declaring income to tax office
But, the concern I'd have is her dishonesty, that she stole £50 and by not contacting you afterwards to say you left out two lots sorry I accidentally picked up your money, she is hoping to get away with it.
If she doesn't return the money or assure you she sees it as next weeks wages, then I'd report her to the police for theft. If she's stealing from you how many other people is she stealing from, and has she taken other items from your house? I'd find it hard to trust her. I had sad experience where we found out my cleaning was stealing from me, hundreds of pounds and some of my nice stuff. It was things like this that tipped me off . Nice as anything to my face 