Usually I've dealt with that by blocking out ALL identifying information from the drawings - including the client and project's name, location, and the name of the firm. Of course the firm at which you're interviewing is probably going to know what firm this work is from, and if it's high profile enough they may also know what project it is. But blacking out this information is usually enough of a statement that you're sensitive to these issues.

If you're aware that there are confidentiality agreements that absolutely prohibit using images of the project then of course you shouldn't use them. But most such agreements don't restrict firms to that extent. They may restrict the publication of the images, or they may not allow you to use the name and location of the project. But they aren't usually so specific that the firm can't show drawings to potential clients and such, except in cases of high-security facilities.

A potentially bigger issue here is using files and images without your current firm's permission. If you're interviewing discreetly then you probably haven't talked to your employer about what materials you can and can't use. All drawings, photographs, digital files, etc. are the property of the firm and they don't have to let you use any of it (and this is spelled out in most state boards' statutes. Anything that you take without permission is theft.)
The AIA's official position on this is that member firms must allow current or recent employees to take samples of their work, including drawings and photos but not digital files. The firm has the right to select which images you take, and they can also charge you for copies of photos, drawings, etc.

Most employers seem to expect that their employees will help themselves to copies of a representative sampling of projects they've worked on. But if you want to be completely above board about things, you might want to start a conversation with your current employer about "updating" your portfolio. Tell them you're interested in keeping a record of all the projects you've worked on and ask if it is alright to make yourself some half-size sets, copies of images, etc.

I think blacking out client and project names might be a good idea anyway.

At an interview I had a couple months back, several of the projects I showed were for clients that the firm I was interviewing at had done previous work for. The previous work was far enough back that I had no idea, but it was pretty obvious that they were shaken up by the fact that the clients went to someone else for the new spaces. In the end it wound up looking like I was rubbing their previous "firings" in their faces, and I was left with major foot-in-mouth disorder.

The funny thing was, I could never imagine anyone passing over the firm I was interviewing at for the firm I was working for at the time. Whole different league in so many ways.

Do you have a job interview, BlueSteel?