- Others do not gain access to the content of the message
In the US amateur radio service, transmitting “messages encoded for the purpose of obscuring their meaning” is prohibited by §97.113 and specifically for spread-spectrum (SS) by §97.311. I would imagine other jurisdictions have similar rules which prohibit such transmissions and/or require that hopping patterns etc. be published.
I'll leave the detailed question of whether it is useful for this purpose when legal to others who are more familiar with the technical aspects of spread-spectrum modulation. Some remarks from general principle follow.
- Others cannot determine the location of the sender
No method can give you this as an absolute guarantee. Someone can always take a wide-band receiver and a directional antenna and look for where the received power is higher. All that spread-spectrum gets you here is requiring the receiver to have a wider bandwidth, therefore being unable to filter out unrelated signals from other radiators, making it harder to find the transmitter at a longer distance.
- Others cannot block message channels
Spread-spectrum techniques increase the power requirements of jamming, because the jammer must decrease the SNR sufficiently across the entire bandwidth and not just a narrow band.
- Others do not know a message is being sent at all
This falls into the same category as point 2; as you move away from the transmitter, a spread-spectrum transmission will apparently disappear into the noise floor sooner, but if you are close enough a transmission is obvious no matter what the frequency.
For the “accidental observer” as you put it, not attempting to decode spread-spectrum transmissions, they will receive the SS transmission as a (possibly imperceptible) increase in the noise floor. Thus, spread-spectrum signals are indeed unlikely to be accidentally received (or interfered with).