The audio cable tracer isn't going to work at all. And given the ambient noise level
on ship (at least when the engines are running) I can see why an audio beeper won't
do much either (though it might be a useful backup in some cases - the sonar might
pick it up.)
A strobe would have the advantage that it could also be used if someone goes
overboard or has to signal an aircraft. But whatever you carry it has to be something
that is turned on whenever one is in the compartment, and a strobe can be
annoying when it is continuously reflecting off of the surfaces around you while you
are trying to work.
Some sort of RF transmitter is probably the best approach, though be forewarned that
finding one in a hold full of metal debris takes some practice. It could also allow you
to open a hatch and "sniff" the compartment to see if someone was in there.
To make that approach work you'd probably want something in the microwave
frequency range, though 433 MHz might work if you could tolerate the larger
antenna on the search receiver. The beacon itself can be quite small - it would
depend on the desired battery life and what other features you wanted to put in the
same case. (The watchband transmitters used for Project Lifesaver run for about
3 months continuously, and can be heard up to 1/2 mile away in a clear area.)
If you look around for designs for WiFi antennas on the web you'll find lots of them,
typically of two types: directional antenna, or omnidirectional ones for base stations.
The directional ones (typically a horn, helix, reflector, bent wire of some sort, etc.)
will give you an idea of the size antenna you might need if you used 2.4 GHz. Here
are some examples:http://www.drivebywifiguide.com/TetraBrikHowTo.htmhttp://helix.remco.tk/http://www.mikestechblog.com/joomla/component/content/article/42-wifi-wireless/58-extend-wireless-wifi-network-building-24-ghz-cantenna.html
These typically run 2 to 6 inches in diameter and 8" to 18" long, depending on
the specific design, the materials used, and the desired gain. There are some other
options as well, but you'll get the idea. The receiver could be built into the back end
of some of these to make a single unit. You don't want too much
enough to enable you to resolve a direction within 30 degrees or so.
For the same gain, the antenna size decreases with frequency. That's why small
directional antennas are more practical on the higher microwave frequencies (such
as 2.4 GHz.) Of course, if you have a WiFi access point in a particular compartment
you may pick up that instead!
The receiver then needs to have some way to indicate the relative signal strength,
because what you have to do is turn the antenna until the signal is strongest, in
which case you should be pointing in the direction that the signal is coming from.
(Which might not be the shortest direction to the source if it is bouncing off the
wall of a compartment, but it's the best you can do - you'll just find that the
direction changes as you approach the wall.) Something like an LED bargraph
display may be adequate, though one potential problem is trying to squeeze
enough display range to allow you to indicate the direction both when the signal
is weak (the other end of the hold) or just a couple of feet away. There are
several ways around this, such as a manual RF gain control (requires more
training to use), a display with lots of resolution (which isn't easy, especially
if you need to read it under difficult conditions), or some sort of automated
range indicator (as is done on the VK3YNG unit.) I like the audio indicator
because it means I can look where I'm going or where the unit is pointing
rather than at the unit, though it may be difficult to use in a noisy environment
even with headphones.
Now I don't know of any ready-made direction-finding (DF) receivers for
the microwave range. There may be some designed for finding WiFi networks,
but most of those probably will run from a laptop computer, and you don't
want to have to carry one of those with you. The circuitry isn't difficult to
design, and you may be able to convert some equipment from another
purpose by adding a signal strength indicator to it. But you'll want to run
some tests to see what sort of coverage you can get and how much power
you need, then design the overall system from there.
There are some other types of DF units that use loops, a pair of antennas, and
some other methods, and I don't think any of them will work well in your
situation due to the large number of reflections from all the metalwork.