Have you considered occult pericardial effusion? I have diagnosed about six unexpected pericardial effusions in the last year, and I have learned a few lessons:
1) this is not as rare a diagnosis as you think
2) patients can look really good, even with impending tamponade
This is an easy diagnosis to miss, but in 2013, I don't think that's acceptable anymore. Any well-trained emergency physician should be thinking about it and has a tool at hand to make the diagnosis quickly and cheaply.
The first video here is in the subxiphoid view.
The next video is the same patient, showing the effusion in the parasternal long.
The third video is from a different patient. Notice here how the right ventricular wall is collapsing inward at end-diastole. This is an echocardiograhic sign of tamponade. For the clinician, tamponade should be a clinical diagnosis, but if you see this at the bedside, be worried!
Occult pericardial effusion is not hard to diagnose. You just need to think of it, and be liberal in your use of PoCUS. Unexplained dyspnea, weakness, hypotension, or tachycardia? Get out your probe!