Look, the problem with all these detection systems is signal to noise
ratio. And all three (radar, sonar and IR) have real bad situations detecting a low floating object. Under the right circumstances they will all show the container, but those circumstances are not common, so what you see is a lot of noise
with occasional blips of signal rising above the noise level. If an attentive watchstander is peering at the display 24/7 a useful warning might result, but if one depends on in-built alarms in the instruments the chance of detection is low.
Thus, for the typical short handed cruising boat, reliable detection of a floating container is dependant upon daylight and an alert lookout. In other situations the low statistical chance of hitting one is all that protects you. Fortunately, this seems to be pretty effective!