How much tread depth is still there? Are the tires wearing evenly? Honestly, I'd have a dealer look at them. 50,000 miles is a lot of driving, and the tires have undoubtedly taken their share of abuse (I don't mean abuse, more like normal wear and tear) and may be structurally worn out. It could also be a defect, or they are the actual wear indicators starting to show up. Still, the fact that it is on all 4 tires, in several spots around the tread indicates to me that they are wear indicator bars as described above.
Let's assume that they are the tread wear indicators for a second here. Are they actually at the point that they are even with the tread, or is there still tread above the "bubbles"? If they are even with the tread, they are pretty much worn out and should be replaced. If they are still below tread depth, then you still have wear left on the tires.
Also, the fact that they are only on the outer edge of the tires incidates an alignment problem, probably toe-in or possibly a camber angle problem. Do you rotate them regularly? If you have an earlier model with the Twin Traction beam suspension up front, they are notorious for uneven wear. You may have plenty of tread towards the center and inner part of the tire, but be worn down towards the outer edge. Alignment problems can cause serious wear problems.
BTW, a tire worn beyond the indicators doesn't automatically signify that it will fail internally (though the odds go up as a tire puts on mileage), but it will be more prone to hydroplaning and won't have nearly the traction of a newer tire.