I take it that you only see this when you are performing thermal mapping studies on steam related equipment such as Autoclaves or Sterilizers. Or you are performing Steam In Place (SIP) of process equipment such as Bio-rectors and filter sterilizations?
If this is the case then what you are seeing is steam condensate (water). The steam condensate tracks back along the inside of the thermocouple sheathing as the condensate/steam is under pressure within the chamber. The water finds the easiest route out of the chamber via the TC sheathing and ends up in the SIM where the Thermocouples are terminated.
To prevent water from entering the data logger terminals you can add drip-cuts to the TC sheathing approximately 1m from the Kaye Validator 2000 data logger. Exercise good housekeeping by placing paper towels or a container under the cuts to collect the condensate.