How to detect overheating? In which part or subsystem the detectors are installed?

Non-Ferrous metal is non-magnetic metals (copper, aluminum, brass, lead, etc.) It will take approximately 50% more of a non-ferrous metal to be as detectable as a ferrous metal. Manganese is also a non-ferrous metal and difficult for most metal detectors to detect. Elenco Metal Detector With Beep Alert Review Detectorly

Non-Ferrous metal is non-magnetic metals (copper, aluminum, brass, lead, etc.) It will take approximately 50% more of a non-ferrous metal to be as detectable as a ferrous metal. Manganese is also a non-ferrous metal and difficult for most metal detectors to detect. Best Underwater Metal Detector In depth Guide Reviews For 2018

no
it cannot detect plastic guns or 3d made guns unless the bullet is in the gun
It is called a "metal " detector

Look at the detector you removed, and all others that may still be wired in.
They all have battery backup. Although the detectors are hardwired, the chirping is coming from one of the detectors to let you know to change the battery.

In most cases, one metal detector will do it all except for specialized situations. So nugget detectors and underwater ones wont be good for more geneal detecting.

Yes, metal detecting under water is actually quite popular. There are a few types; Snorkeling with a metal detector, Scuba detecting and Shipwreck diving.

All of these types require equipment that can be expensive to hire/ buy.

SC No. Item Definition Possible Cause Related
SCs Troubleshooting Procedure Type
SC 221 Synch.
detection signal
error 2
221-001: Y
221-002: M
221-003: C
221-004: K
Main scan length detection is
not properly completed ten
consecutive times.
The front (for C&M) or rear (for
K&Y) laser synchronizing
detector boards are used for
the main scan length
detection, which automatically
corrects the main-scan
magnification.
• Damaged or
disconnected cable
between front (C&M)
or rear (K&Y) laser
synchronizing
detector board and
the LD unit
• Incorrect installation
of front (C&M) or rear
(K&Y) synchronizing
detector board (the
beam does not target
the photo detector.)
• Defective front (C&M)
or rear (K&Y)
synchronizing
detector board
• Defective LD unit
After doing any of the following, print ten
jobs or more to see if the same SC code
is displayed:
1. Turn the main switch off and on.
2. Check or replace the cable
connecting front (for C&M) or rear (for
K&Y) synchronizing detector board
and the LD unit.
3. Check or reinstall the front (for C&M)
or rear (for K&Y) synchronizing
detector board.
4. Replace the front (for C&M) or rear
(for K&Y) synchronizing detector
board.
5. Replace the laser optics housing unit.
6. Replace the BCU.
If a synch. detector board cannot be
replaced, do the following as a
temporary measure:
• Disable main scan length detection

It is a radar detector detector. So if you are running a radar in a state where it is illegal, this technology can detect the presence of your radar detector and you can be pulled over and ticketed. This is done by detecting the local oscillators and other "RF leakage" from the radar detector.
Sometimes you may notice that when you are near another car with a radar detector, your radar detector may go off. There are situations where your own radar detector can detect leaking RF energy from another detector in a nearby car.

IF this happens when you first get into the hot car, the problem is likely to be that the electronics in the unit are too hot. If the problem goes away after the car cools off inside (assuming you have AC), then your unit has just become overheated.

The speaker's voice coil likely has overheated, and now it is rubbing on the insides of the speaker magnet. This causes very garbled sound. All Sunkyong Early Warning radar detectors provide horrible radar detection performance. You should seriously consider getting a much better radar detector.