Where have my scientists gone?
Go to the Stash and click gadgets. You may have the nerd Repellant active (which means they won't appear); turn it off and they should walk around from now on.
You are using the Nerd Repellent. Removing scientists from the map is the whole point of that gadget.
Simply equip a different gadget from the Stash.
Easy enough to solve, just unequip or replace the Nerd Repellent gadget.