That does sound odd and suspicious. [] A couple of other possibilities:


Is there sufficient contrast in your subject? If a monochromatic leaf is larger than the AF point, it's like trying to focus on a blank wall - not enough contrast. The AF points do extend a bit beyond the boxes seen in the viewfinder, but they are smaller than the spot metering circle.


What AF mode are you in? If you're in AI-Servo, there is no focus lock confirmation beep/light. If you're in AI-Focus and you or the subject are moving (vine leaf in the wind would do it), the camera will use Servo mode and again there is no focus lock confirmation. But, if you're in One Shot it should lock as long as there is sufficient contrast under the AF point.