I think the requirement or test case was coming from you with heading "Re: recovery tricks"
Here's the email snippet:
  (d) held ESC+Refresh down until they saw the Recovery screen

There are not timing requirement when going into recovery mode. Is this specific to Oak family ? 

The current official instructions on www.google.com/chromeos/recovery say "press and hold Esc + Refresh, then press Power. Let go of Power, then let go of the other keys." -- they do not specify any timing details. Thus, unless we want to change that text, we should probably try to test a range of timings to cover as much possible user behavior as we can.
 
It is a good question, though, why this wasn't found during the more exhaustive manual/FSI testing?

The firmware test team performed 4 rounds of firmware testing on Elm, 2 on EVT and 2 on DVT.   As stated above we have various boot sequence tests.  AC connected is not part of the test case.  Should with and without AC be added as part of the test case ? Is this new to oak/elm and most likely on future boards ?  

Yes, since the Silego is tied to both recovery and battery cut-off on all current designs (although I'm not sure if that will change with Haven), it would probably be worth adding a more extensive "entering recovery" test case. Something like:

Perform each of these both with and without AC connected and verify that you end up on the recovery screen:

1. Hold down Esc+Refresh, then tap Power and let go of all keys as fast as you can (~100-300ms is probably humanly possible).
2. Hold down Esc+Refresh, then hold down power and release all keys after at least one full second.
3. Hold down Esc+Refresh, then hold down power and keep all keys held until the recovery screen appears.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/vboot_reference/+/00d4be66721b24903f977e770148179035254e19

commit 00d4be66721b24903f977e770148179035254e19
Author: Edward Hill <ecgh@chromium.org>
Date: Tue Oct 02 20:23:08 2018

Ignore power button if held on startup

Ignore a power button push until after we have seen it released,
to avoid shutting down immediately if the power button is held
down on startup.

BUG=b:116819414, chromium:670492 
BRANCH=grunt
TEST=manual:
1) Press and hold esc+refresh+power.
2) Depthcharge shows INSERT screen and does not power off.
3) Release esc+refresh+power.
4) Press and release power.
5) Depthcharge powers off.
TEST=test_that --fast -b grunt $grunt_ip firmware_ECLidShutdown
TEST=FEATURES=test emerge-grunt --nodeps vboot_reference

Change-Id: I7421a4b1a1b8a7894f0e7d1c7927ffc52d9faac0
Signed-off-by: Edward Hill <ecgh@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1256023
Reviewed-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

[modify] https://crrev.com/00d4be66721b24903f977e770148179035254e19/tests/vboot_api_kernel2_tests.c
[modify] https://crrev.com/00d4be66721b24903f977e770148179035254e19/firmware/lib/vboot_ui.c

Thanks for the fix!

The following revision refers to this bug:
  https://chromium.googlesource.com/chromiumos/platform/vboot_reference/+/5536c751eda9e729c95310443f81767c13fd125e

commit 5536c751eda9e729c95310443f81767c13fd125e
Author: Edward Hill <ecgh@chromium.org>
Date: Fri Oct 05 02:11:10 2018

Ignore power button if held on startup

Ignore a power button push until after we have seen it released,
to avoid shutting down immediately if the power button is held
down on startup.

BUG=b:116819414, chromium:670492 
BRANCH=grunt
TEST=manual:
1) Press and hold esc+refresh+power.
2) Depthcharge shows INSERT screen and does not power off.
3) Release esc+refresh+power.
4) Press and release power.
5) Depthcharge powers off.
TEST=test_that --fast -b grunt $grunt_ip firmware_ECLidShutdown
TEST=FEATURES=test emerge-grunt --nodeps vboot_reference

Change-Id: I7421a4b1a1b8a7894f0e7d1c7927ffc52d9faac0
Signed-off-by: Edward Hill <ecgh@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/1256023
Reviewed-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>
(cherry picked from commit 00d4be66721b24903f977e770148179035254e19)
Reviewed-on: https://chromium-review.googlesource.com/c/1263029
Reviewed-by: Martin Roth <martinroth@chromium.org>
Tested-by: Martin Roth <martinroth@chromium.org>

[modify] https://crrev.com/5536c751eda9e729c95310443f81767c13fd125e/tests/vboot_api_kernel2_tests.c
[modify] https://crrev.com/5536c751eda9e729c95310443f81767c13fd125e/firmware/lib/vboot_ui.c