The power brick approach is an improvement but still makes the following assumptions:
* location of network ports is at floor level hidden under desks
* power and network cable colours match
* an employee won't disconnect a seemingly useless box when they need to charge their phone
* port security is not in use
A replacement "trojan horse" computer or printer that has been modified externally is a stealthier approach. Such devices have a reason for being connected to a power source and the network and do not raise suspicion (especially if the replacements are soiled and have worn asset stickers attached). Local IT staff will ensure the devices have network connectivity and will likely assume (in the case of a computer) that suspicious network traffic is the result of a virus.
Failing that full blown approach, even a "signal booster" could be a better disguise. An average person will think of their analogue TV and radio signal boosters. Further disguise can be added by soiling the devices, attaching asset stickers and stickers for a matching fake brand name and fake website where suspicious users can have their fears alleviated. The website has the added benefit of alerting the attacker that their device has been potentially compromised.