Could be wickback, or maybe she has spilt something?
You should be able to sus it if she has spilt something though, a ph check should help aswell i.e. if you know you left the carpet at a particular ph level, if it comes up acidic then she may have spilt a drink (if the drink was acidic of course).
I would bite the bullet and check. If its wickback then use something to not put moisture into the backing i.e. saphire/cfr handtool, or even blot with your preferred choice of cleaning agent.