WEP is worthless. Really, and truly, if someone wants to look at your "WEP protected" traffic, it's almost trivial to do so. Switch to WPA. It is truly secure.
As spice points out, just not broadcasting your network name (the standard calls it a Service Set ID or SSID) will not "hide" your network. You need several more security measures to ensure that YOUR network is safe from the appartment complex and that it is most definitely NOT theirs.
First, as I pointed out, switch to WPA security. That will make your traffic secure. Then use MAC address filtering. This allows only certain wireless devices to join your network, and does so by checking their unique hardware addresses (the Media Access Controller address or MAC address). This is almost entirely foolproof. Combined with WPA, your network will be solidly secure and impervious to the apartment complex's actions-and your neighbors' attempts to get FREE wireless. You should also make sure your network is on a different channel from the complex's-as far different as you can make it. Most access point devices come out of the box set for channel 6 or 7, so go for channel 1 or 11 to start.