You could do a number of things that are more subtle to establish ways for the investment to not be noticed, by moving the money around, etc., but I expect that your main concern would be how to fly under the radar while compounding works.
In fact, perhaps people in the time-travellers world who want to invest into their future would be chiefly concerned that time travellers in their future are coming back and dominating all the investments without ever plowing the proceeds back into their present ..