Strange NTC behaviour: it goes open circuit when cooling down
This is a quite common behavior of NTCs, i.e. Negative Temperature Coefficient thermistors: probably the device you've at hand is designed to work at high temperatures, therefore its resistance is very high at ambient temperatures, so high that for a normal tester it resembles an open circuit (for example \$\ge 10\mathrm{M}\Omega\$). Here it is a typical resistance vs. temperature behavior diagram taken from the Wikipedia commons:



The vertical asymptote can start even at \$25°\mathrm{C}\$, making the increase of resistance at low temperature very steep.