Hiding what? You have to assume that whatever device you make will get taken apart. Security through obscurity? Security through obscurity - Wikipedia, the free encyclopedia
I suppose it is always possible to separate the die from the plastic package and inspect the die for the temp sensor, but If you removed the identifying labeling on the uC, the fact that it had a thermal sensor in the uC's packaging would be rather difficult to determine.
Replacing any set number in a RNG algorithm with the number pulled from the analog temperature sensor would add complexity.
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