I don’t think there are any signs at the bank. The sign of a brumating turtle, is that you spot the turtle, or you detect it by telemetry, or by other means.
If you care about the turtle, then shining a light on it is probably not a great idea.
Brumation, like surviving winter for many animals, is a gamble that the animal’s energy reserves will last longer than deceiving warm spells, harassment by predators (like dogs chasing deer), etc. Every calorie counts, so even minor stressors could be the difference between life and death, especially during a long or harsh winter.
If you make a hole in the ice, then you are also drawing attention to the location, which could lead to more harassment.
Yes, it is unadvisable.