Surveillance and espionage are nothing new. A long time before the current technologies existed, there were already numerous methods to gain information more or less legally.
Perhaps the easiest way to get proprietary information has always been to steal it. As long as this information is stored on a medium that can be removed, it is not protected against somebody taking it.
Some people always exist that know information that is not stored anywhere or that is out of reach. In that case, it may be promising to black mail a person over using the service of a prostitute who may have been hired for that purpose. A variant of this involves an attractive woman or man who forms a close personal relationship with an employee with access to the desired secrets.
If this doesn't help, it might be possible to hire a competitor's employee who has valuable knowledge, or to bribe a supplier or employee.
Finally, if it is not possible to get the desired information from somebody who is part of the organization an agent can try in infiltrate this organization.