Their are two cases.
Case 1: error marker appears but no error is listed in problems view

1. Create a new worksheet.
2. add an incorrect line of code : 1++ 1
3. the marker with an error appears : "value ++ is not a member of Int"
4. No error is listed in the problems view

Two things to note:
a) If we click on the worksheet the error a appears in the bottom margin in red (see screen shot A)
b) when we save and compilation proceeds an error does seem to appear and then quickly disappear from the problems view

Case 2: some errors are simply not reported

1. Create a new worksheet.
2. add an abstract and concrete class (see screen shot B)
3. override a 'def' but use a name that is not in the base class
4. No error is listed in the problems view (see screen shot B)
5. No error marker appears

Note that in this case I cannot see if the error actually appeared and then disappeared from the problem's view
If we click on the worksheet no error appears in the bottom margin in red as in the first case

Scala IDE version:
4.0.0.rc2-2_11-201410301039-a7c06b2
Scala compiler version:
2.11.4.
Scala library version:
2.11.4.
Eclipse version:
4.4.0.v20140925-0400
Class loader store: 1 entries
Compiler v. 2.11.4.(/home/hmf/dev/scala/eclipse/plugins/org.scala-lang.scala-compiler_2.11.4.v20141023-110636-d783face36.jar)